As a provisional system of terrestrial digital television broadcasts and digital terrestrial sound broadcasts, the broadcast systems referred to as the wideband ISDB-T system and narrowband ISDB-T system have been proposed. These broadcast systems are systems having mutual compatibility, construct OFDM (orthogonal frequency division multiplexing) modulated basic transmission unit referred to as “segment” in a bandwidth (about 429 kHz) obtained by dividing the frequency band of 6 MHZ allocated to television channels in Japan into 14, and use the segments for digital terrestrial television broadcasts or digital terrestrial sound broadcasts.
The signal of the segments is OFDM modulated. As the number of OFDM carriers of a segment, three modes of 108, 216, and 432 are defined. In digital terrestrial television, the transmission signal is comprised by using 13 segments, but in digital terrestrial sound broadcasts, it is determined by the provisional system that the transmission signal be comprised using one segment or three segments.
The OFDM carriers in a segment are modulated by an identical modulation scheme. As the modulation scheme, DQPSK, QPSK, 16QAM, 64QAM, etc. are defined. Carriers in a segment include not only carriers transmitting information, also various types of pilot signals, a transmission control signal, etc. As the pilot signals, there are a CP (continual pilot) and SP (scattered pilot), while as the transmission control signal, there is a TMCC (transmission and multiplexing configuration control) signal. Also, as additional information, there are signals such as AC1 (auxiliary channel 1) and AC2 (auxiliary channel 2). The CP and SP among the pilot signals are BPSK (binary phase shift keying) modulated by a PRBS (pseudo-random binary sequence) output corresponding to the carrier number. Also, the additional information AC1 and AC2 are BPSK modulated in the OFDM symbol in a frame header by the output of the PRBS corresponding to the carrier number similar to the pilot signals CP and SP, but are differentially BPSK modulated in the following OFDM symbols by the additional information to be transmitted with reference to the phases of the additional information AC1 and AC2 in the OFDM symbol in the header of the frame. Also, the transmission control signal TMCC is BPSK modulated in the OFDM symbol in the frame header by the PRBS output corresponding to the carrier number similar to the additional information AC1 and AC2, but is differentially BPSK modulated in the following OFDM symbols based on the information of the transmission control signal TMCC with reference to the phase in the OFDM symbol in the frame header.
In the wideband ISDB-T system, a signal is comprised by 13 segments, but while PRBS using the same generator polynomial is used, the system is set so that the initial value given to the circuit generating the PRBS differs according to the number of each segment and is configured so that there is no contradiction in phases of pilot signals CP at the upper ends and lower ends of adjoining segments. The initial value given to the circuit for generating the PRBS is made different according to the location of the segment in this way for the purpose of randomizing the phases of the pilot signals CP and SP in each segment as much as possible so as to prevent a peak from occurring in the wideband ISDB-T signal and reduce a dynamic range of the signal.
FIG. 5 shows the configuration of a segment of a digital terrestrial television broadcast system, that is, the wideband ISDB-T system, and the phases (represented by the number in each block) of the various types of pilot signals CP and SP, transmission control signal TMCC, and additional information AC1 and AC2. Note the phases (and representative numbers) can be contrasted with those in a narrowband ISDB-T signal as shown in FIG. 6.
As illustrated, in the wideband ISDB-T system, the phases of the pilot signals CP and SP, transmission control signal TMCC, and additional information AC1 and AC2 in each segment are controlled to be random. For this reason, the occurrence of a peak in a signal of the wideband ISDB-T system can be prevented and demand on the dynamic range of the receiver can be eased.
According to the above digital terrestrial television and sound broadcast systems, the frequency bands used for the broadcasts will be the frequency bands of analog type terrestrial television broadcasts being actually broadcast at present. For example, the UHF band allocated to television broadcasts at present is scheduled to be used as the frequency band for digital terrestrial television broadcasts, while the VHF band allocated to television broadcasts at present is scheduled to be used as the frequency band for digital terrestrial sound broadcasts. For this reason, it is considered that in the VHF band allocated to digital terrestrial sound broadcasts, at least the present channel structure will not change until analog television broadcasts shift to digital. Namely, in digital terrestrial broadcasts as well, the broadcast service will be started based on the present television channels. Due to this, it is considered that in digital terrestrial sound broadcasts, signals will be configured based on 6 MHZ (4 MHZ).
In the narrowband ISDB-T system used in digital terrestrial sound broadcasts, signals of a one-segment format and three-segment format are defined. Due to this, there is only one segment number in the one-segment format and only three in the three-segment format. FIG. 6 shows the segment configuration of a narrowband ISDB-T signal and the phase relationships of the various types of pilot signals. As illustrated, when all of the signals in a channel are one-segment signals, all of the 13 segment numbers become identical, therefore if the initial value given to the circuit for generating the PRBS is set in accordance with the segment number, the initial values become identical and consequently the phases of the pilot signals CP and SP of all of the 13 segments become identical. Also, the transmission control signal TMCC and the additional information AC1 and AC2 at the time of no modulation similarly become the identical phase in all of 13 segments. For this reason, when viewing the overall signals in the channel, there are a large number of groups of carriers having aligned phases, therefore the probability of occurrence of peaks in the transmission signal becomes high and there is a disadvantage that it becomes difficult to secure the dynamic range of a front end amplifier in the receiver.
Therefore, when transmitting a narrowband ISDB-T signal, it can be considered to control the phase of the carrier depending upon the frequency position in each transmission channel so as to suppress an increase of the dynamic range of the broadcast signal. For example, one may modulate sub signals such as pilot signals and transmission control signal by using the PRBS generated by using the initial value set in accordance with the frequency position of the broadcast channel and for example OFDM modulate the modulated signals together with the encoded main signal to thereby generate a broadcast signal. By employing this method, the system is set so that the phases of the main signal and sub signal carriers for the different channel are different, so the dynamic range of the broadcast signal can be suppressed and the demands on the dynamic range of the front end amplifier in the receiver can be eased.
Corresponding to the above broadcast apparatus, in a digital broadcast receiving apparatus, unless signal processing corresponding to the signal processing performed at the broadcast apparatus side can be suitably carried out with respect to the received broadcast signal, information source data contained in the encoded main signal cannot be correctly reproduced or the sub signals no longer can be correctly reproduced, whereby there arise disadvantages that the control information contained in the transmission control signal cannot be correctly taken out and it becomes impossible to correctly receive the digital broadcast.